(1) Field of the Invention
The present invention relates to imaging devices. In particular, the present invention relates to devices that detect and provide a measure of an intensity of light impinging on imaging pixel devices.
(2) Background Information
Imaging arrays are frequently used to produce an image representing an object. The imaging arrays are typically formed of rows and columns (bitlines) of photo detectors (pixels) that generate photo charges proportional to light reflected from an object to be imaged. Photo charges from each pixel are converted to a signal (charge signal) or potential representative of a level of energy reflected from a respective portion of the object. The signal or potential is read and processed by video processing circuitry to create an image representing an object.
Pixels belongings to a same bitline are usually connected at a common output node from where a signal or potential, representative of the level of energy, is read out. Pixels belonging to the same bitline "see" an overall capacitance (hereinafter referred to as "bitline capacitance"), at the common output node. Each pixel in a same bitline is individually controlled to read out at the common output node. Typically, pixels belonging to a same row are commonly controlled by a same signal such that an entire row may be read out at a substantially same time.
Improvement in the speed at which rows of pixels are read out, without substantially negatively affecting bitline swing, constitutes a constant challenge for circuit designers. To meet the increasing need for high speed image sensor devices, it has become necessary to integrate image sensor arrays with digital circuitry that controls the operation of the array and processes the array's output. Integration of an image sensor with complementary-metal-oxide-semiconductor (CMOS) support circuitry is most desirable because of the low power consumption characteristics and common availability of CMOS technology. Such an imaging array integrated with a CMOS support circuitry is called CMOS active pixel sensor (APS) array.
Typically, a pixel cell includes a driving device that receives an electronic signal indicative of an intensity of light detected by the image sensor and drives a current proportional to the measure of intensity, to a bitline to which the pixel cell is coupled. Following signal integration, the pixels in a selected row are accessed by asserting a WORDLINE signal to each pixel access device of each pixel cell of the selected row. Then the bitlines to which the pixel cells of a same selected row are coupled may be charged by a current driven by a driver device of the pixel cells of the selected row, to a voltage level representative of an intensity of light detected by the pixel cells of the selected row. The pixels of an entire row may thus be read out at a substantially same time. The pixel cells of other rows, not currently accessed, have their pixel access devices switched off by a deasserted WORDLINE signal corresponding to these rows.
In case a previously accessed cell, coupled to a bitline, is at a higher voltage than the voltage of a currently accessed cell coupled to the same bitline, the bitline needs to be "reset", prior to a read out of a currently accessed cell coupled to that bitline. In this way, when a currently accessed pixel cell is read out, the bitline is charged to a voltage that faithfully reflects a measure of an intensity of light detected by the currently accessed pixel. It is desirable to provide an apparatus and method that ensure that, before a pixel (row) is read out, a bitline to which that pixel belongs is prepared to accurately reflect, after that pixel is read out, a measure of the intensity of the light detected by the pixel. Moreover, it is desirable that such preparation is performed in a fast manner that does not cause delay in the read out of the respective pixel cell.